Integrated apparatus and process for high recovery of acetylene from the reaction of calcium carbide with water

ABSTRACT

An apparatus for the manufacture of acetylene and hydrated lime from the reaction of calcium carbide and an excess of water is disclosed. A primary reactor for the initial reaction of the calcium carbide and water is disposed concentrically within a secondary reactor. Hydrated lime and unreacted calcium carbide flow directly from the primary reactor to the secondary reactor, where the carbide lime settles from the water and is removed.

RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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FIELD OF THE INVENTION

This invention relates to an apparatus, control, and process for theproduction of acetylene and hydrated lime by the reaction of calciumcarbide with water.

BACKGROUND OF THE INVENTION

In a typical wet process for manufacture of acetylene from calciumcarbide, particles of calcium carbide are introduced to an excess ofwater in a reactor vessel on a continuous or semi-continuous (on/off)basis. Water is added continuously to the reactor and acetylene and ahydrated carbide lime slurry are withdrawn from the reactor on acontinuous basis. A system is provided for stirring the contents of thereactor to mix the calcium carbide with the water and to maintain amore-or-less uniform slurry of hydrated carbide lime. Because thereaction of calcium carbide with water is exothermic, the temperaturemust be controlled, typically by the rate at which fresh water is added.A greater rate of water addition results in cooler temperatures and alesser rate of water addition results in warmer temperatures.

There are several undesirable characteristics of the process of theprior art. These in summary are:

1. Some acetylene yield is lost through the premature discharge ofunreacted calcium carbide.

2. Some acetylene yield is lost through the solubility of acetylene inthe large volume of water passing through the reactor.

3. Operational difficulties occur due to large, solid inert particlesthat enter the system and interfere with the stirring mechanism anddischarge pumps and valves.

4. The overall process efficiency is reduced because a low concentrationof hydrated lime in the discharge stream results in low contact times inthe reactor and poor use of the reactor space.

5. The hydrated carbide lime value is reduced due to the presence ofgranular impurities and variable hydrated lime concentrations.

These characteristics are described further as follows:

Discharge of Unreacted Carbide (breakthrough)

In a typical reactor configuration the reaction kinetics areapproximated by a Constant Flow Stirred Tank Reactor (CFSTR). Accordingto Levenspiel, Chemical Reaction Engineering, 2nd Edition, John Wileyand Sons, Inc. 1972, Chapter 5, page 97. " one type of! idealsteady-state flow reactor is called the mixed reactor, the backmixreactor, the ideal stirred tank reactor, or the CFSTR (constant flowstirred tank reactor) and, as the name suggests, it is a reactor inwhich the contents are well stirred and uniform throughout. Thus theexit stream from this reactor has the same composition as the fluidwithin the reactor. We refer to this type of flow as mixed flow, and thecorresponding reactor, the mixed reactor, or the mixed flow reactor."Most, if not all, commercially practiced wet acetylene processesapproximate the CFSTR configuration.

In the CFSTR there is unreacted carbide that is mixed throughout thereactor. The particle size of the unreacted carbide varies from larger,recently introduced particles to smaller less-recently introducedparticles that are nearing the completion of reaction. In an ideal CFSTRsome of these smaller unreacted particles will be discharged with thehydrated lime slurry. Any acetylene generated by a particle after it hasbeen discharged from the reactor may be lost to the atmosphere. Thepremature discharge of unreacted particles is referred to as"breakthrough" and, if the reaction kinetics are known, the extent towhich breakthrough occurs can be estimated through calculations about anideal CFSTR.

Practical evidence as well as theoretical calculations of CFSTR kineticsshows that this breakthrough can be significant. While the actual amountof breakthrough is affected by the particle size of the carbide feed,the hydrodynamic behavior of a carbide particle reacting to formacetylene, the internal configuration of the reactor and the inherentreactivity of the calcium carbide, the amount of breakthrough increasesas the space velocity of the reactor increases. Space velocity isdefined conventionally as the number of reactor volumes displaced in onehour.

FIG. 1 is a graph showing the mathematical relationship between thespace velocity and breakthrough, assuming ideal behavior and relying onpublished data for CFSTR kinetics. For reactors of space velocitiesequaling four or greater (4000 liter/hr. in FIG. 1), the idealconversion is about 96% or less, which means that the breakthroughlosses are greater than 4% of the acetylene that is produced from agiven carbide feed. Most commercial reactors in operation today operateat a space velocity greater than four, which means that the breakthroughlosses are even greater.

Dissolution Losses

Another problem with traditional technologies is the losses of acetyleneto dissolution in water. FIG. 2 shows the solubility of acetylene inwater as a function of temperature for three commonly operatedpressures. It may reasonably be assumed that the water of the hydratedlime discharge slurry is saturated in acetylene, and unless this wateris recycled to the system, all of the acetylene dissolved in the waterwill eventually be lost to the atmosphere. The amount of acetylenecontained in the discharge hydrated lime slurry can be calculated byknowing the amount of water exiting the reactor and its outlettemperature and pressure. Even if some of the water is recycled to thereactor the open vessels which serve as settling tanks to thicken thehydrated lime are exposed to the atmosphere and a large portion of theacetylene so dissolved is lost.

To illustrate, if pure calcium carbide (MW=64) is reacted to formacetylene (MW=26) with enough water to result in a 5% hydrated lime(MW=74) slurry, the amount of water flowing from the outlet per Kg ofacetylene produced will be: ##EQU1## Assuming that the outlettemperature is 50° C. and the reactor pressure is 0.3 atm-gauge (atypical set of conditions), the amount of acetylene contained in 54 Kgof water is 0.09% or 0.05 Kg. Thus about 5% of the acetylene generatedis lost to the atmosphere through the dissolution in water. This loss isin addition to the acetylene lost as a result of breakthrough ofunreacted particles.

Operational Difficulties

Another problem with conventional technologies pertains to theoperational difficulties created when non-reactive materials areintroduced along with the calcium carbide. These unreactive materials,which are present in all commercially available calcium carbidematerials, usually comprise inert coke, solid ferrosilicate and othermetallic or mineral particles. These are materials introduced with thelimestone or coke fed to the furnaces that manufacture calcium carbideand are carried through to the final product. These inert materialsaccumulate in the reactor and if not removed will eventually interferewith the stirring mechanism and discharge pumps or valves, causingmechanical breakdown. Smaller particles that do not rapidly settle arecarried through to the settling tanks where they may accumulate, causingdifficulties with the discharge system.

Loss of Process Efficiency

Another problem of traditional systems is that the concentration ofhydrated lime in the reactor is kept low, usually below about 10 weightpercent, to reduce premature settling of hydrated lime, i.e., to keepthe slurry of the hydrated lime precipitate in a free-flowing state. Ifallowed to settle the hydrated lime would result in plugging ofdischarge lines or create unmanageable accumulations of hydrated lime inthe reactor, or both. Low hydrated lime concentrations also result fromthe manner in which temperature is controlled. In a typical process,temperature is controlled at 50° C. If the temperature begins to rise,the usual procedure is to increase the rate of water feed, which bringsdown the temperature, but also has the undesirable result of furtherdiluting the hydrated lime output stream. In hot weather, where heatlosses to the environment are reduced, and for very large systems, thehydrated lime concentration in the output stream can fall to as littleas 3 weight percent.

The operational requirement for low concentrations of hydrated limerequires larger reactors for a given space rate or a higher space ratefor a given reactor size. Larger reactors require greater capital costs.Higher space rates result in greater breakthrough. Low hydrated limeconcentrations also result in a greater per unit water throughputincreasing the losses due to solubility. The combined losses due tothese effects are typically 8 to 12%, which means that only 88 to 92% ofthe acetylene produced by a given calcium carbide is recovered. Finally,the resulting hydrated lime discharge stream must be sent to largerholding vessels in order to provide adequate residence time for thehydrated lime to thicken for subsequent use or disposal. These vesselsoccupy more land and require the need for additional capital investment.

Loss of Calcium Hydroxide Product Value

Particulate impurities contained in the calcium carbide are carried intothe hydrated lime product unless they are filtered out at some expenseand operational effort. These small particle impurities may adverselyaffect the value of the hydrated lime for downstream utilization. Ingeneral, the value of hydrated lime improves if the granular impuritiesare reduced in both size and quantity. The value of hydrated lime alsoimproves with concentration and with consistency in concentration. Mostdownstream uses require the transportation of the hydrated lime and thegreater the concentration, the more Ca(OH)₂ is transported per ton ofslurry. In commercial practice of the prior-art, little if anything isdone to segregate small particle impurities. In addition, processes toincrease the concentration of the hydrated lime are separate from thereactor and add additional capital and operating costs.

Objects of the Invention

An object of the present invention is to reduce the aforementionedproblems of a) loss of acetylene due to breakthrough, b) loss ofacetylene due to solubility, c) operational difficulties due to inertmaterials, d) loss of process efficiency due to low hydrated limeconcentrations, and e) loss of hydrated lime value due to particulatesand low, or variable, hydrated lime concentrations.

It is also an object of the present invention to provide an apparatusand a process for reacting calcium carbide with water that provides ahigh level of recovery of the acetylene produced.

It is also an object of the invention to improve the level of recoveryof acetylene in the reaction of calcium carbide with water by reducingor eliminating breakthrough and minimizing the acetylene dissolved inthe water discharge stream.

It is also an object of the invention to provide an apparatus and aprocess which allows for high completion of reaction and lowbreakthrough of calcium carbide in a given reactor space.

It is also an object of the invention to provide an apparatus andprocess which provides low losses due to dissolution of acetylene.

It is also an object of the invention to provide an apparatus and aprocess which segregates large particles from small particles therebyimproving the operability of the process and the value of theco-produced hydrated carbide lime.

It is also an object of the invention to provide an apparatus and aprocess which improves the process efficiency by improved utilization ofreactor space.

It is also an object of the invention to provide an apparatus and aprocess which provides for control functions which allow for flexibleacetylene output while maintaining a steady-state hydrated limeconcentration and constant temperature.

It is also an object of the invention to provide control of the pressureof the apparatus while allowing variable acetylene production andsteady-state concentration of hydrated lime and constant temperature.

It is also an object of the invention to provide a process controlstrategy which provides for maintaining the preferred temperature andhydrated lime concentration conditions while varying acetylene output.

It is also an object of the invention to provide an acetylene/calciumhydroxide production process in which the calcium hydroxide product isuniform in quality (particle size, purity and concentration).

It is also an object of the invention to provide an apparatus andprocess in which the hydrated lime product can be recovered from thereactor at high Ca(OH)₂ concentrations.

It is also an object of the invention to provide an apparatus andprocess which minimizes the requirements for internal moving parts andas a consequence simplifies the construction and operation of theapparatus.

It is also an object of the invention to provide an apparatus andprocess which minimizes the number and size of ports, external fittings,and moving seals through the pressure vessel, and as a consequencesimplifies the construction and improves the overall safety andoperability of the apparatus and process.

Further objects of the invention will become evident in the descriptionbelow.

BRIEF SUMMARY OF THE INVENTION

Overview

The present invention is an improvement of the apparatus and processdisclosed in U.S. Pat. Nos. 5,082,644 and 5,284,630 to Bunger et al.(Bunger et al.), which are hereby incorporated by reference. The Bungeret al. system comprises two successive reaction steps. In the first stepparticulate calcium carbide is charged into an entrained or ebullatedflow primary reactor containing water. The water in the primary reactoris directed upward to ebullate calcium carbide particles. The calciumcarbide particles react with the water until their size becomes smallenough to become entrained and carried upward in the upward flow ofwater. These particles of unreacted calcium carbide and the solidcalcium hydroxide reaction product entrained in the upward water floware conveyed through a conduit to a separate secondary reactor tocomplete the reaction of the calcium carbide particles under plug flowconditions. The calcium hydroxide produced in the primary and secondaryreactors is allowed to settle in the secondary reactor and is recovered.Acetylene gas generated in both reactors is drawn off above either orboth reactors.

The Bunger et al. reaction system is continuous. The calcium carbidefeed can be metered in a controlled manner into the reaction system. Thecalcium hydroxide is typically much purer than that produced byconventional acetylene processes, because the entrained flow in theprimary reactor serves to effectively separate out particulateimpurities introduced with the calcium carbide, such as oversized cokeand mineral impurities, which are usually are not entrained and carriedinto the secondary reactor. Thus, the calcium hydroxide product has ahigher market value and can be used as a feedstock for the production ofhigh-value calcium derivatives.

Apparatus and Process of the Present Invention

The present invention utilizes the same dual reaction system as theBunger et al. system. However, in the present invention, the entrainedflow primary reactor is positioned within the secondary reactor,preferably in a concentric manner with the axis of the two reactorscoinciding. Thus, the present invention has the same advantages as theBunger et al. system. However, the placement of the primary reactorwithin the secondary reactor materially increases the control over thereaction, eases construction and reduces costs, augments theefficiencies of the system, and improves the product purity.

The apparatus of the present invention is a single-vessel, two-stagereactor system for producing acetylene by reaction of calcium carbideand water. In the first or primary reactor, calcium carbide iscontinuously charged into an ebullated and entrained flow reactionenvironment containing an excess of water. The primary reactor isdisposed concentrically within secondary reactor, such that thesecondary reactor surrounds the primary reactor in an annular fashion.The unreacted water with unreacted calcium carbide and calcium hydroxideproduct from the primary reactor flows over an overflow (e.g., weir orports) at the perimeter of the primary reactor directly into thesecondary reactor. The secondary reactor is configured as a dense phase,laminar plug-flow reactor where calcium hydroxide product is allowed tosettle and is removed from the bottom of the reactor. Most of thecalcium hydroxide settles, but a minor portion may remain in suspensionin the water, that is withdrawn from the secondary reactor over anoverflow weir and recycled to the primary reactor.

According to Levenspiel (1972), " the other! of the two idealsteady-state flow reactors is variously known as the plug flow, slugflow, piston flow, ideal tubular, and unmixed flow reactor, . . . Werefer to it as the plug flow reactor and to this pattern of flow as plugflow. It is characterized by the fact that the flow of fluid through thereactor is orderly with no element of fluid overtaking or mixing withany other element ahead or behind. Actually, there may be lateral mixingof fluid in a plug flow reactor; however, there must be no mixing ordiffusion along the flow path. The necessary and sufficient conditionfor plug flow is for the residence time in the reactor to be the samefor all elements of fluid."

The majority of the reaction occurs in the primary reactor ebulated bedreaction system (essentially CFSTR), and is then transferred to aplug-flow reaction system in the secondary reactor. The plug-flowreaction environment in the secondary reactor allows for substantialcompletion of the reaction of calcium carbide with water. The advantageof any plug flow reactor is that a reaction can be completed in thesmallest possible reactor space. Further, there is no mixing ofunreacted materials with that element in the reaction system which hascompleted its reaction.

Acetylene is drawn off as it is generated in the primary and secondaryreactors. Calcium hydroxide formed by the reaction is separated in thesecondary reactor by settling and is removed from the dense zone at thebottom of the secondary reactor.

A suspension of unreacted calcium carbide particles and calciumhydroxide is transferred between the primary reactor and the secondaryreactor over a concentric overflow weir at the periphery of the primaryreactor. Since the suspension flows directly from the primary reactorinto the secondary reactor, the need of any cross-over piping iseliminated. With no cross-over piping the possibility of plugging of apipe and the heat loss from the surface of the piping is eliminated. Thetwo-stage, single vessel reaction system is also mechanically simpler,and thus less expensive to build and maintain. In the secondary reactoritself no mechanical mechanism is required, such as rakes. However, itis contemplated to use such rakes if desired. The internal placement ofthe primary reactor within the secondary reactor permits a much smallerfootprint than separate placement of the two reactors.

Another advantage is that only one pressure vessel need be built. Thewalls of the primary reactor are only required to separate it from thesecondary reactor. Since there is virtually no pressure differentialbetween the reactors, the construction of the primary reactor may be oflight-weight design. The pressurization for the entire reaction systemis maintained by the outer walls of the reaction vessel, requiringconstruction and certification of only one pressure vessel.

The concentric internal placement of the primary and secondary reactorsallows a common header at the top of the pressure containment vessel tobe used for gas collection for both the primary and secondary reactors.The result is a significant overall reduction in gas volume in thereaction system, as compared to separating the two reaction systems intoseparate pressure vessels. This increases the volume available forreaction, and increases safety by decreasing the volume for potentialadverse and undesirable reactions.

Control of the Process

Temperature

The concentric construction of the primary reactor within the secondaryreactor allows for an unprecedented control of the process, measurablygreater than in conventional systems and the dual reactor Bunger et al.system. Since the primary reactor is contained within the secondaryreactor, there is a much lower heat loss from the primary reactor ascompared to a separated reactor construction as disclosed in the Bungeret al. patent. Most of the heat of reaction is generated in the primaryreactor, and instead of losing heat to the external environment, it isinsulated from the external surfaces of the reactor vessel by thesurrounding secondary reactor. Thus, heat leaves the primary reactormainly by vapor generation, by reactant/product flow into the secondaryreactor and through the wall separating the reactors. Thus, thesecondary reactor can be maintained at a higher temperature to decreaseacetylene solubility. Temperature control of the primary reactor iseased, since the secondary reactor is usually at a higher temperaturethan the external environment and heat lost from the primary reactor outthrough the primary reactor walls to the secondary reactor is minimizedsince the secondary reactor temperature differs slightly from that ofthe primary reactor.

Basically, the present system allows for retention and recovery of muchof the reaction heat produced in the primary reactor where most of theheat from the calcium carbide/water reaction is produced. The lower heatloss allows for better temperature control in the primary reactor andoperation at a higher reaction temperature. The secondary reactor mayalso be operated at a higher temperature, because of the highertemperature of the primary reactor and heat added from the primaryreactor. The higher reaction temperature is favorable as it lowers thesolubility of acetylene in water, reducing the reservoir ofunrecoverable acetylene dissolved in the water in the reaction systemand allowing it to be recovered as the desired product gas.

The process temperature may be conveniently controlled by adjusting theloss of heat to the surroundings. Because of the concentric, heatconserving dual reactor design, temperature is conveniently controlledby conserving the heat generated in the system, if a higher temperatureis required, or dissipating heat to the environment, if cooling isdesired. Heat conservation may be accomplished, for example, by heatexchanging the slurry output with the water inlet. Dissipating heat tothe environment or may be accomplished, by cooling the water recycledbetween the secondary reactor and the primary reactor. This means oftemperature control is a significant departure from conventionalpractice which uses the water input as a means of temperature control.Making temperature control independent of the water input flow rateallows for independent adjustment of the water input flow rate tomaintain a predetermined hydrated lime slurry concentration.

Control of Hydrated Lime Product Concentration

By practice of the present invention, the concentration of Ca(OH)₂ inthe hydrated lime slurry removed from the system can be set atessentially any convenient concentration, even at high concentrationswherein the hydrated lime is settled and in the form of a semisolidslurry. In principle, the hydrated lime concentration can be as high asdesired, and is limited in practice only be its flow characteristics.This contrasts with conventional systems which require operation in aregime of an agitated free-flowing and dilute liquid slurry for theremoval of the slurry. In the present invention, there is a secondaryreactor in which the hydrated lime is allowed to settle into a thickenedslurry that is free of particulate impurities that would seriouslyinterfere with its easy removal from the reactor.

The hydrated lime slurry, by the present process, can be produced at aconsistent composition, i.e., with a consistent water concentration.This consistency of composition along with the removal of impurities inthe entrainment system, results in a purer, higher-value product.

The high concentration of hydrated lime and its corresponding lowerconcentration of water allows for many advantages. The absolute amountof hydrated lime produced is set by the acetylene demand, because it isstoichiometrically related to the amount of acetylene produced. If theconcentration of the hydrated lime is high, that dictates that theabsolute amount of excess water used in the system is lower. The loweramount of water reduces the amount of water that is available fordissolving acetylene. As indicated above, a significant loss ofacetylene is due to acetylene becoming dissolved in the excess waterthat passes out of the system. Reducing the water in the system byincreasing the hydrated lime concentration materially reduces theselosses.

Another advantage of the reduction of water flow through the system isthat the space velocity is reduced for the same size reactor, whichreduces the possibility of breakthrough, i.e., calcium carbide beingcarried through and out with the water/lime output. By the practice ofthe invention, with its reduction of dissolved acetylene and thesignificant reduction in breakthrough, theoretical calculations showthat 99 percent, or more, of the acetylene available from the calciumcarbide feed can be recovered.

Another advantage of the reduction of water flow through the system isthat the reactor size can be reduced for the same space velocity. Thisreduces the size of the reactor and as a result reduces the capitalcosts. The use of a plug-flow reactor regime results in a more effectiveuse of the available space than for CFSTR reactors. Total internalvolumes of less than 1/3 of conventional reactor volumes are possiblewithout an increase in breakthrough.

Another advantage of reducing the water input is that the heat loss fromthe system through the water is reduced. A substantial amount of theheat is lost from the system through the heated excess water leaving thesystem. This limits the operating temperature of the system. By reducingthe amount of water passing through the system the operating temperaturecan be increased, which reduces the losses from acetylene dissolved inwater since the solubility of acetylene is reduced.

A high lime-concentration slurry can be handled in the present inventionbecause of the higher quality of the hydrated lime product. Inconventional practice there is no classification and separation of solidimpurities. Thus, if the hydrated lime product were removed in asettled, thickened state from the bottom of a conventional CFSTRreactor, it would contain significant amounts of "rocks", which wouldcreate severe operational problems. Accordingly, the hydrated lime isremoved in conventional systems as a free-flowing suspended slurry, andthe rocks that settle are periodically removed from the bottom of thereactor by a separate means. In contrast, in the present invention theseimpurities are removed in the primary reactor entrainment system,leaving the slurry in the secondary reactor essentially free thereof.Operationally, this allows slurry removal through a simple meteringpump, throttling valve, or other appropriate means. Thus, the rate ofremoval of the hydrated lime, and hence the hydrated lime concentrationin the water/lime slurry and the amount of water throughput, can easilybe controlled. Basically, the only upper limitation to the concentrationof the hydrated lime in the slurry is the viscous effects that occurwhen the concentrations of the hydrated lime exceeds about 40 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing conversion of calcium carbide and breakthroughin a prior-art continuous stirred tank reaction system.

FIG. 2 is a graph showing the solubility of acetylene in water.

FIG. 3 is a cross-section of a reactor system of the invention.

FIG. 4 is a schematic of an apparatus of the invention incorporating areactor as in FIG. 3.

FIG. 5 is a schematic showing a preferred material control method of theinvention.

FIG. 6 is a schematic showing an alternate material control method.

FIG. 7 is a schematic showing a temperature control method.

FIG. 8 is a graph showing the percent of acetylene produced which islost through solubility as a function of hydrated lime concentration.

FIG. 9 is a graph describing the kinetics of the reaction of calciumcarbide with water, showing conversion parameter v. time.

FIG. 10 is a graph showing settling velocity of calcium hydroxide withrespect to calcium hydroxide concentration in terms of weight percent.

DETAILED DESCRIPTION OF THE INVENTION

Apparatus

Reference is made to FIGS. 3 and 4, which illustrate an apparatus of theinvention. The apparatus is comprised of a main reactor vessel 101designed to withstand internal operating pressures according toregulatory standards. Within the cylindrical portion of the reactorwalls is fitted a cylindrical weir wall 102 which top provides acircular weir 103 over which water/slurry may flow. At the bottom of theweir wall and between the weir wall and the reactor wall is fitted aplate 104 at such an angle to the horizontal so that water/slurry willbe directed to the low point of the plate 105.

Between the weir wall 102 and a primary reactor 109 is fitted a baffle106 which top elevation 107 exceeds the top elevation 110 of the primaryreactor 109 and which bottom elevation 108 extends below the elevationof the circular weir 103. The top elevation 110 of the primary reactor109 is intermediate between the top elevation of the circular weir 102and the top elevation 107 of the baffle 106. The baffle 106 is used todirect the flow of Ca(OH)₂ and unreacted calcium carbide in a downwarddirection toward the secondary reactor plug-flow reaction zone 111 andto provide a quiescent environment settling of Ca(OH)₂ into a dense zone125.

The primary reactor 109 may be fitted with a rotatable screen 113 (shownin FIG. 3 in its vertical, discharge position and in FIG. 4 in itsoperating horizontal position) in the bottom portion or bottom elevationof the primary reactor 109 which supports the bed of calcium carbide.This screen 113 may be rotated about a horizontal axis to allow fordischarge of any unreactive material that has been retained on thescreen 113.

Below the primary reactor is fitted a velocity control restrictor 114which supplies water/slurry to the primary entrained flow primaryreactor 109 and allows for discharge of unreactive particles. Thevelocity control restrictor 114 originates in a disengagement zone andholding tank or zone 147 which allows for separation of inert particlesfrom the slurry recycle.

The plug-flow reaction zone 111 of the secondary reactor 121 isconfigured as a simple cone 115 which angle 116 is greater than theangle of repose of settled hydrated lime. Alternatively, the cone may befitted with a slow-moving rake (not shown).

The top of the reactor is fitted with a port 117 for introducing calciumcarbide, a port 118 for discharge of acetylene and appropriate overpressure relief devices 119. Make-up water is introduced to the systemat a top water inlet port 120 and is introduced to the vapor space byany convenient distribution method. The top inlet port 120 mayoptionally include sprayers as shown.

The primary reactor 109 is sized to be sufficiently small in diameter asto provide maximum settling time in the upper part of the secondaryreactor and to minimize the mass flow rate of the recycle stream neededto ebulate the reactor bed. Conversely, the primary reactor is sized tobe sufficiently large in diameter so that the reactor operates in abubble-flow regime and not a slug-flow regime. The height of the primaryreactor is sized so as to contain a sufficient volume for controlledreaction of the instantaneous steady-state inventory of calcium carbide,but not so large that space is inefficiently used.

The baffle 106 is sized large enough to reduce the downward velocity ofthe liquid overflowing the primary reactor wall but small enough toprovide long residence time in the rising portion 112 of the secondaryreactor 121. The annulus between the weir wall 102 and the main reactorvessel wall is sized to be sufficiently large to accommodate the flow ofrecycle water, but not so large that it reduces the available spaceneeded for the baffle 106 or the primary reactor 109.

In the illustrated design, the only internal movable part is the screen,and this part only moves occasionally during cleaning. Thus the onlymoving seal in the pressure vessel is that required for the screen axis,which is not required to operate during normal operation of the system.This contrasts with conventional CFSTR designs that have manypenetrations of the pressure vessel with moving seals, such as forrakes, grates, etc., which function continuously through the process. Inaddition, the mechanical energy for the process of the present inventionis supplied by an external pump 142 that drives the recycle stream,which requires no drive shafts penetrating the reactor vessels. Thereduction of moving seals in the pressure vessel materially simplifiesthe mechanics and increases the reliability of the system.

Process

In the process of the invention calcium carbide is introduced to theprimary reactor 109 by any suitable calcium carbide feeding means 141through the calcium carbide feed port 117. The calcium carbide reactswith water in the primary reactor 109. The water is recycled by pump 142from the secondary reactor 121 through a water recycle pipe 143, andintroduced to the primary reactor from the bottom through the velocitycontrol restrictor 114 to create a reaction in an ebullated environmentwith water flowing up through solid particles. Large particles settle tothe level of the screen 113 where they react until they become smallenough to become entrained by the upward velocity of water in theprimary reactor 109 when they are carried over into the secondaryreactor 121. Particles of impurities settle through the screen or areperiodically discharged (by rotating the screen 113) and allowed tosettle through the velocity control restrictor into a holding tank 147fitted at the bottom. The screen is rather coarse, e.g., about 10 mm orcoarser, so that only large particles are retained on the screen. Theholding tank 147 is periodically emptied through a discharge port 129.

Small particles of unreacted carbide, Ca(OH)₂ and inert materials arecarried over the top 110 of the primary reactor 109 after which theyflow downward by the force of gravity into the annular space 122 betweenthe primary reactor 109 and the baffle 106. The downward momentum of thefluid and its entrained particles passing over the primary reactor wall110 so that the particles drop to the elevation of the water, (asdetermined by the elevation of the circular weir 103) assures that theparticles are completely submerged in water, thereby reducing oreliminating the accumulation of dry particles on the surface that mayoccur when feeding calcium carbide of small particle size.

Calcium hydroxide and any unreacted calcium carbide are allowed totravel downward between the annular space 122 between the primaryreactor and the baffle, where they begin to settle into the dense zone125 of the secondary reactor. A portion of the slurry travels under thelower elevation 108 of baffle 106 and rises in the space for the risingportion 112 of the secondary reactor 121 between the weir wall and thebaffle where it overflows the top of the weir wall. Water/slurryoverflowing this weir wall is drained to the lowest point 105 where itexits the reactor vessel and is recycled through line 143 to the bottomof the velocity control restrictor.

The material introduced through the recycle line 143 to the bottom ofthe primary reactor 109 through the velocity control restrictor 114serves as the mixer in the primary reaction, creating the flow of waternecessary for the ebullated reaction environment, and the classificationof particles in the velocity control restrictor 114. The recycle ratealso affects the hydrodynamic behavior in the primary reactor 109 andthe upper portion of the secondary reactor 121 and provides a means fordistributing heat from the primary reactor to the secondary reactor.While the recycle is primarily used to recycle water, the material inthe recycle loop may include in addition to water, suspended solids suchas calcium hydroxide, and other solids. Using any appropriate screeningapparatus 123, the recycle loop may serve the purposes of screening fromthe system any lighter-than-water debris that may inadvertently beintroduced to the system, since such debris will be retained in theloop.

Below the lower elevation 108 of the bafffle 106 and the top 124 of thecone the hydrodynamic behavior undergoes a transition to a conditionapproximated by Plug Flow Reaction (PFR) kinetics, which has beendefined above. For first order chemical reactions the ideal PFR is theonly reactor which, in theory, will allow for (very nearly) completereaction within a finite reaction space. Thus, the apparatus of theinvention provides a reaction environment in the lower portion of thesecondary reactor which approximates that of a PFR and as a result, intheory, allows for virtually complete reaction and as a consequence, thealmost total elimination of breakthrough.

Settled and concentrated calcium hydroxide is discharged from the bottomof the cone through line 128 to a hydrated lime containment area. In theprocess of the invention the reaction is performed under conditionswhich maximize the concentration of slurry. The segregation of largeparticulates in the primary reactor allows for thickening and settlingin the secondary reactor without concern for operational difficultiesthat occur with CFSTR reactors. This allows for convenient operation ofa throttling valve in the exit stream that can operate at any convenientpressure.

Control

Mass Flow Control

The preferred mass flow control strategy is shown in FIG. 5. Thedownstream demand for acetylene dictates all other process flows. Thedownstream demand is sensed by the process control 201 which controls acalcium carbide feeding means 203. An optional flow measurements device207 is used to measure acetylene production rate and may provideinformation to supplement the downstream demand information.

The same information used to control the carbide feed rate is sent tothe hydrated lime slurry valve controller 204 which controls the rate atwhich the hydrated lime slurry is discharged by means of a meteringvalve 205. The rate at which hydrated lime slurry is discharged may be afixed ratio of the calcium carbide fed, thereby establishing a fixedconcentration of slurry.

An optional density/mass flow meter 209 provides information to thecontroller about the actual flow rate and density of the slurry.Information regarding the density of the slurry may be used for finetuning of the metering valve 205.

Fresh water is added through water inlet ports 120 to maintain theliquid inventory at a constant level in the reactor system. The reactorsystem is fitted with a level detection device 211 of any convenientdesign. An electronic differential pressure device is one such means andthis device monitors the level of fluid in the weir annulus 171. Asignal from the level detection device is sent to a controller 213 whichcontrols the flow of fresh water by means of a throttling valve 215. Ifthe level of fluid falls below a predetermined level, the levelcontroller 213 farther opens the throttling valve 215 to direct waterthrough. Conversely, if the level of fluid rises above a predeterminedlevel the level controller 213 further closes the metering valve 215 torestrict the flow of water through 120. The level of water in the weirannulus 171 is thus maintained at a level preestablished by the desiredset-point on the level detection device 211.

In the process of the invention the rate of water addition is controlledto maintain a steady inventory of fluid in the system and is aconsequence of control of other process variables. Combined with asteady-state flow of calcium carbide to the reactor this allows for asteady-state concentration of slurry from the reactor. By prescribingthe desired concentration of the outlet slurry the rate of discharge canbe precisely adjusted to achieve that concentration and the remainder ofthe flow parameters, specifically the water flow, are automaticallyadjusted accordingly.

An alternative mass flow control strategy is shown in FIG. 6. In thealternative strategy the carbide feed rate means 303 is controlled by acontroller 301 sensing downstream demand, as before. An optional flowmeasuring device 307 is used to measure the production rate and may beused to fine tune the control.

In the alternative strategy, a predetermined flow rate of water isestablished based on the anticipated amount for reaction losses,humidity losses and free water desired in the slurry. An optional flowmeter 306 is used to provide information to the controller 305 as to theactual flows.

In the alternative strategy the inventory of fluid within the reactor isdetected by a level detection device 312 and set to a prescribed levelin the weir annulus 371. The level is influenced by the rate ofdischarge of hydrated lime slurry controlled by a device 309 controllingthe hydrated lime slurry outlet valve 311.

Temperature Control

Temperature is controlled by adjusting the heat losses to theenvironment. If it is required to recover heat, as would occur for lowerconcentrations of hydrated lime, colder feed water or a colderenvironment, heat can be recovered at either of two convenient points.These are the hot acetylene gas and the slurry outlet stream. Heatrecovered from either of these streams can be reintroduced into thefresh water feed by a feed/effluent heat exchanger 151 or 153 shown inphantom in FIG. 4 (or in FIG. 7 showing recovery from the slurryeffluent). Water heated from the hydrated lime slurry effluent in 153may be introduced directly into the reactors as in FIG. 4, or as in FIG.7, mixed with unheated water through a mixing valve 181, which iscontrolled by a controller 183 that monitors the temperature of theprimary reactor by temperature sensor 185.

For highly concentrated calcium hydroxide outlet streams with acorrespondingly lower water throughput or for warmer feed watertemperatures or warmer environments, the operating temperature may riseabove regulatory limits if a means of cooling is not provided. Underthese circumstances, the temperature is maintained below regulatorylimits by a controlled release of heat to the environment. This can beaccomplished by lowering or eliminating the heat recovery at exchangers151 and 153. Heat may also be released to the environment by cooling therecycle stream by a suitable heat exchanger 155. (See FIGS. 4 and 7.)

The preferred temperature control strategy is shown in FIG. 7. Thedesired temperature is sensed 185 in the primary reactor and a set pointis selected, not to exceed regulatory limits. The controller 183controls the proportion of fresh process water 154 which passes througha feed/effluent heat exchanger 153 and that which does not pass throughthe heat exchanger. This temperature control loop is operationalwhenever the desired temperature is higher than the operatingtemperature.

When the operating temperature is higher than the desired temperature,the system may be cooled by withdrawing heat from the recycle loop orwater reycle pipe 143 by means of a heat exchanger 155 cooled by coolingwater 156. A controller 184 controls the rate at which cooling water 156passes through the heat exchanger 155 by means of a throttling valve157.

In effect, temperature is controlled by conserving or dissipating heatbetween the reactor and its surroundings. The strategy allows forcontrol of temperature without having to influence any of the processflow rates. Specifically, maintaining a constant system temperature doesnot require adjustments in any of the process flow rates, even when thecarbide feed rate varies or when there are uncontrolled temperatureeffects (changes in fresh water temperature, changes in the temperatureof ambient surroundings, etc.). Thus, the process of the inventionprovides for unusually steady-state operation in all major processvariables. Most importantly, a steady-state of calcium hydroxide outletconcentration can be achieved and a steady-state operating temperatureis maintained even when the rate of acetylene and calcium hydroxideproduction is varied.

This uncoupling of the temperature control and mass flow rate control isa significant departure from the prior art in which temperature and massflow rates are highly coupled. By uncoupling the two requirements, bothtemperature and mass flow rates can be independently controlled foroptimum performance without the need to compromise one or the other.

Safety Control

Appropriate safety devices are designed to shut down the system in theevent of over- or under-filling, for over-temperature, forover-pressure, or for loss of fresh water or recycle flow. Additionallyprecautions are taken against under-pressure that may occur when a lowpressure operation is shut down and cooled. All customary engineeringprecautions are taken to ensure safe operation in the event of partialor total failure of electrical or mechanical components.

Preferred Process Conditions

In the process of the of the invention, calcium carbide is fed to theprimary reactor and the reaction is allowed to proceed under temperatureand pressure conditions which maximize the recovery of acetylene. Bothtemperature and pressure have a significant influence on theconcentration of acetylene dissolved in the outlet water. The solubilityof acetylene in water as a function of temperature and pressure is givenin FIG. 2, which shows the solubility at various pressures gauge (1 atmambient pressure). The graph shows that the lowest solubility occurs athigh temperatures and low pressures. In the preferred mode of operation,the reaction between calcium carbide and water is carried out at thehighest temperatures allowed by regulation and the lowest pressureacceptable as an inlet pressure for the downstream apparatus. Followingthis principal, it would be desirable, from a thermodynamic point ofview, to operate the system at pressures below atmospheric (vacuum) andsuch operation is within the spirit of the invention. Practical problemswith potential ingress of air and regulatory standards may limit theactual pressure conditions of the process to something greater thanatmospheric pressure, preferably 0.02 bar or greater.

In the process of the invention the loss of acetylene due to solubilityin the discharge water is due to two factors, the amount of acetylenedissolved in a unit of water (or otherwise retained in the slurryoutput) and the number of units of water discharged per unit ofacetylene produced.

Simple mass balance calculations show that the number of units of waterdischarged per unit of acetylene generated is directly reflected in theconcentration of hydrated lime in the outlet. FIG. 8 illustrates thepercent of the acetylene manufactured which is lost through solubilityas a function of hydrated lime concentration operating at three typicalpressures and at thermally adiabatic conditions and for a calciumcarbide feed that is 80% CaC₂ and 20% CaO (a typical composition). Asseen from the graph, as the hydrated lime concentration increases, thelost acetylene decreases.

Accordingly, preferred operating conditions are at higher hydrated limeconcentrations, at lower pressures, and higher temperatures. Usingmathematical modeling, an acetylene recovery of 97.2% can be achieved atsecondary reactor temperature of 70° C., a pressure of 1.3 bar, and 14wt. % hydrated lime concentration. This recovery can be increased to99.8% by a secondary reactor temperature of 90° C., and a pressure of0.05 bar, and a hydrated lime concentration of 40 wt. %. In summary, toachieve an acetylene recovery greater than 97%, the process of theinvention is operated at a secondary reactor temperature above 70° C., apressure between 0.02 and 1.3 bar gauge, and a hydrated limeconcentration between 14 and 40 wt. %.

Rate and Thermal Effects

Details of the invention are further understood in view of the rate andthermal effects.

Rate Effects

Reaction Kinetics--Reaction kinetics refer to the rate of reaction ofcalcium carbide with water. This rate depends on the starting size ofcalcium carbide particles, pH of water, but not much on temperature orpressure. Rates may also vary in unknown ways with carbide sources fromdiffering calcium carbide manufacturing processes.

FIG. 9 illustrates the kinetics of calcium carbide reaction with water.Here the conversion parameter (ξ) is defined as; ##EQU2## where C₂ H₂ isthe amount of acetylene evolved at time (t) and (C₂ H₂)₀ is the totalamount of acetylene that can be evolved. Defining a conversion parameteris this fashion is a way of linearizing the rate of evolution ofacetylene from a three-dimensional shrinking calcium carbide particle asit reacts with water.

As shown from the graph of FIG. 9, which is derived from actual data,the reaction is initially quite rapid as shown by the steep slope of theline. This is following by a regime exhibiting much slower reactionkinetics. The present invention utilizes the two reactor design toexploit this kinetic behavior. In the primary reactor, the initial rapidreaction stage takes place, along with a rapid generation of heat. Thereacting of the particle in the slower regime does not occur until afterthe particle has been substantially reduced in size and has been carriedinto the secondary reactor. The secondary reactor has a long residencetime. Because most of the reactor space is used to perform thissecondary reaction and because a plug flow reaction regime ismaintained, the calcium carbide particles can become fully reacted, inspite of the fact that their reaction rate is slow when they are in thesecond reaction regime.

Settling Velocity--This rate effect addresses settling velocity ofcarbide particles, calcium hydroxide particles and inert impurities inthe original carbide. The settling velocity depends on particle size,density of the particles, density of the fluid, and viscosity of thefluid. In the case of calcium hydroxide, the settling rate also dependson the settling effects in a slurry, which are highly complex and noteasily described mathematically. The larger the particle, the moreimportant are the bulk slurry properties, the smaller the particle, themore important are the surrounding fluid properties. Particle size maybe influenced by the carbide manufacturing process. Particulates ofalumina have significant effects of settling velocities because theycreate hydrates which interfere with settling at the microscopic andmolecular level. FIG. 10 is an illustration of the settling velocity ofCa(OH)₂ versus the weight fraction in the slurry. As shown from thegraph, the settling velocity decreases as the weight fraction increases,which indicates the difficulty of achieving high concentrations ofhydrated lime by gravity settling from dilute hydrated lime suspensionsof the prior-art. However, in the present invention, whereconcentrations of hydrated lime are maintained at high values at alltimes, long settling times are not needed to obtain a product of highconcentration.

Preferably the hydrated lime concentration in the process of theinvention is 14 wt. % or higher. The withdrawal rate for withdrawing theslurry is preferably at a ratio of the rate to the mass flow rate of thecalcium carbide feed less than 8, more preferably less than 5.

Mass Flow Rates--This rate effect considers the rate at which water andcarbide (CaC₂, CaO, impurities) enter the reactor, the rate at whichacetylene and calcium hydroxide are produced by the reactions and therate at which calcium hydroxide and water are discharged from thereactor. The input and output rates are dictated by stoichiometricrelationships and desired slurry concentration. The overall mass balancefor varying operating conditions are discussed in further detail below.

The mass flow rate also includes the rate of recycle even though thisrate does not affect the overall mass balance. Unless heat isdeliberately removed from the recycle stream, the recycle rate also haslittle influence on the overall energy balance. However, the rate ofrecycle is highly important to several operational parameters. Asrecycle rate is increased, the following occur:

1. The ebullation effect in the primary reactor is increased and thesize of particles entrained in the vertical flows increases.

2. The temperature rise from the bottom to the top or the primaryreactor decreases, allowing the temperature in secondary reactor to behigher without exceeding regulatory temperatures in the primary reactor.

3. The size of particle allowed to settle in the velocity controlrestrictor increases.

4. The rise rate of water between the baffle and the weir wall increasesreducing the settling of Ca(OH)₂ and thereby increases the concentrationof slurry in the recycle loop.

5. The size of unreacted particles that might settle in the velocitycontrol restrictor is increased.

6. Consumption of electrical energy is increased.

Conversely, a lower recycle rate has the following effects:

1. The ebullation effect in the primary reactor is decreased and thesize of the particle entrained is decreased.

2. The temperature rise in the primary reactor increases, placing anupper temperature limit on how hot the recycle stream can be.

3. The segregation of particles between the secondary reactor and theprimary reactor results in a cleaner hydrated lime product.

4. The rise rate between the baffle and the weir wall is decreasedleading to settling and lower concentrations of hydrated lime in therecycle streams.

5. The downward velocity between the primary reactor and the baffle isdecreased, resulting in decreased mixing in the secondary reactorplug-flow regime.

6. Consumption of electrical energy is decreased.

The optimum recycle rate will be one which properly classifies particlesbetween the primary reactor and secondary reactor, which allows forconsistent temperature control with an acceptable, higher slurry outlettemperatures, and which allows for an acceptable rate of settling ofCa(OH)₂ particles in secondary reactor. In general, the recycle rateshould be the lowest that will allow for acceptable temperature control.

Thermal Effects

Heats of Reaction--This thermal effect considers the heat released byreaction of calcium carbide with water and by reaction of calcium oxidewith water, coupled with the rates of these reactions. The first effectfrom heat released by reaction of calcium carbide with water isdiscussed above. The second effect from hydration from calcium oxide istaken to be instantaneous and all the heat generated is consideredliberated in the primary reactor, although in practice a small amountmay be carried to the secondary reactor before hydrating.

In adiabatic steady-state operation of the process of the invention theenergy balance equation can be written as:

    accumulation=in-out+generation

where, at steady-state;

accumulation=0

in=sensible heat of carbide feed and water

out=sensible heat of slurry and enthalpy of the humid acetylene stream

generation=heat of reactions

Combining the energy and mass balances allows for mathematical modelingfor the process performance at differing steady-state conditions. Theinterrelationship of process variables is demonstrated in Tables I toVI, for 25 m³ /hour acetylene at a primary reactor temperature of 90°C., feeding an 80% purity CaC₂. The tables provide data for water inlettemperatures of 20° C. and 40° C. and system pressures of 0.05, 0.3 and1.3 bar gauge. The data in these tables quantitatively illustrate therelationship of process variables for an adiabatic system in which noheat is consumed or dissipated. As stated elsewhere, for lowerconcentrations of hydrated lime it is desirable to conserve heat toreduce the acetylene losses. For concentrations higher than those shownin Tables I to VI, cooling will be required. For such cases and for aconstant secondary reaction temperature the lost acetylene is directlyproportional to the amount of free water discharged with the hydratedlime slurry.

Heat Transfer--This thermal effect considers free convective heattransfer, forced convective heat transfer and conductive heat transferthrough solid barriers. All these combine in appropriate ways to provideenergy flows from the primary reactor to the secondary reactor, from thesecondary reactor to the surroundings and from the recycle loop to thesurroundings.

Equilibrium--This thermal effect considers phase equilibria of threespecies in the system that depend on temperatures: the solubility ofcalcium hydroxide in water, the vapor pressure of water above the liquidin the reactor and the solubility of acetylene in the liquid in thereactor. All of these depend on temperature and the latter one dependson system pressure as well.

Process Controls of the Invention

Mass flow rate, liquid level, pressure, and temperatures are monitoredand appropriate actions are taken regarding carbide input, water input,recycle rate, hydrated lime outflow and splitting ratios to heattransfer units. This is guided by safety, production and economicconsiderations.

Flow Rates--There are two input flows, two output flows, and oneinternal flow stream. The choice of the internal flow rates depends onhydrodynamic factors that are described above. The input flow of calciumcarbide is dictated by the output demand for acetylene. The input flowof water is dictated by the desired output Ca(OH)₂ slurry concentration.Therefore, the two output streams, acetylene and hydrated lime slurry,dictate the control over the two inlet streams, calcium carbide andwater, respectively. Table VII shows the relationship of these streamsfor differing acetylene outputs in an adiabatic system.

Liquid Level--The total liquid inventory in the reactor is convenientlyand accurately controlled by monitoring the liquid level in the annulusbetween the weir wall 102 and the outer wall 101 and by using thisinformation to control the flow of water entering the system (see FIG.5, the preferred method). Alternatively, this same information could beused to control the slurry outflow from the secondary reactor (see FIG.6, the alternative method).

Pressure--System pressure may be controlled by any convenient means.Establishing the gas delivery pressure establishes the other systempressure requirements such as slurry outlet, carbide feed, and minimumwater feed pressure. Generally, a lower pressure is preferred, as thesolubility of acetylene in water is lower at lower pressures than athigher pressures. The lowest practical limit is about 0.02 bar gauge. Ifthe delivery system requires a higher pressure, the operatingtemperature may by increased, within regulatory limits, to partlycompensate for the higher solubility of acetylene in water at higherpressures. Losses due to higher solubility may also be reduced byincreasing the hydrated lime concentration in the lime slurry outlet.Higher pressures are often desired in captive acetylene systems to avoiddownstream compressors, e.g., where the acetylene is required at higherpressures for direct use or as a feedstock for another process.

Temperature--Preferably, the temperature is as high as allowed byregulatory standards, which typically limit the maximum temperature tobetween 80° C. and 90° C. Fundamental thermodynamic considerations showthat the solubility of acetylene falls to zero as the water reaches itsboiling point at the prevailing pressure and dissolved solidsconditions. In any case, the temperature does not exceed the regulatorylimits set by safety regulations.

In general, the temperature of the system is increased by lowering heatlosses from the reactor and is decreased by increasing heat losses fromthe reactor. Losses from the reactor occur from three principalsources 1) the enthalpy of the hydrated lime slurry outlet stream, 2)the enthalpy of the humid acetylene outlet stream, and 3) conductive andconvective heat losses to the ambient surroundings. Theoreticalcalculations show that for typical conditions the greatest heat lossesoccur with item 1) and the least with item 3), with item 2) beingintermediate to 1) and 3).

Temperature is most directly increased by decreasing the flow of thehydrated lime slurry outlet stream (by increasing the hydrated limeconcentration). Once the desired mass flow rates and slurryconcentration have been set, a desirable means of temperature control isto regulate the amount of heat losses, and to do so without interferingwith the desired mass flows. This can be accomplished by varying theamount of heat transferred from the outlet streams to the inlet streams.A convenient way to do this would be, for example, to preheat the inletwater using the sensible heat of the hydrated lime slurry outflow.Alternately, the sensible heat of the acetylene stream can be used topreheat the inlet water. If cooling is required, heat may be lost to theenvironment by cooling the recycle stream.

Overall Control Strategy

The control system described above is particularly advantageous wherethe acetylene demand is variable. In most acetylene production systems,the downstream demand for acetylene can vary widely. The acetyleneproduction is increased or decreased by varying the input of calciumcarbide. The desired Ca(OH)₂ concentration can be maintained at aconstant level by increasing or decreasing the slurry out flow tocorrespond with the calcium carbide in-flow. Make-up water forreactions, and losses to humidity and slurry outflow is accuratelyadjusted by maintaining a constant liquid inventory. Temperature iscontrolled by varying the heat conserved from or dissipated to theenvironment.

In an adiabatic system, dissipation to the environment is usually neededfor slurry concentrations above about 20% to maintain the preferredtemperature condition. However, for a non-adiabatic system, theisothermal condition, where no heat conservation or dissipation isrequired, will be a little higher because of uncontrolled heat losses.For practical systems where there are variable heat losses to theenvironment (summer vs. winter temperature difference) or where therecycle rate is fixed and where there are uncontrolled convective andconductive heat losses to the environment, the energy and mass balanceequations suggest that the natural isothermal conditions occur at about20-24% solids in the hydrated time slurry and for reactor pressure of0.05 bar gauge, and feed water temperatures of about 20° C. To achievepreferred temperature conditions for hydrated lime slurries below thisconcentration, heat may need to be conserved. For concentrations higherthan this level, heat may need to be dissipated.

The temperature control strategy is illustrated in FIG. 7, which is morefully describe elsewhere and which shows how the system can be operatedwith constant temperature conditions and constant hydrated lime slurrycomposition under varying acetylene demand.

Illustrations of Controlled Conditions

Tables VIII and IX show for a differing acetylene output stream were therecycle stream is kept constant and the temperature in controlling bycontrolling heat transfer to the environment. Heat transfer to theenvironment is controlled either 1) by conserving heat by heating thewater in stream with the exit hydrated lime stream, which is shown inthe table by a water-in temperature greater than 20° C., or 2) bycooling the recycle stream as shown by a positive value for recycle heatloss.

Table VII is for a calcium hydroxide concentration of 17 wt. % with avariable recycle stream but constant secondary reactor temperature.Table IX is for a calcium hydroxide concentration of 35 wt. %. In thelatter, heat losses to the environment through the exit hydrated limestream are insufficient to maintain the temperature low enough, socooling is required for the recycle stream at all of the acetylenedemands shown. In Table VII, at the lower acetylene demands, heat mustbe conserved to keep the temperature from falling below 90° C.

Design for Optimum Efficiency

Dimensions--Diameters and wall thickness of vessels are specified aswell as diameters and wall thicknesses for piping. Also heights ofvessels and lengths of piping are specified, along with any accessorystructures such as gratings, weirs, etc. These decisions are guided bymass and energy flows and restraints regarding temperatures andpressures. The intents and purposes of the dimensions are discussedabove. FIG. 3 is approximately to scale for system of a capacity ofabout 25 to 50 cubic meters acetylene per hour and shows a pressurevessel 1.2 meters in diameter.

Breakthrough--From previous considerations of reaction rates andsettling velocities, the amount of unreacted calcium carbide leaving thesecondary reactor with the hydrated lime slurry is calculated. Ideallythe secondary reactor is sized so as to eliminate breakthrough. A largersecondary reactor decreases or eliminates breakthrough but adds to thecost of construction and the space required. In general, as long as theprimary reactor is large enough to be operated in the bubble-flowregime, breakthrough will be negligible for any practical secondaryreactor that can be constructed around the primary reactor.

Dissolved Acetvlene--From previous considerations of acetylenesolubility and mass balance, the amount of dissolved acetylene leavingthe secondary reactor with the hydrated lime slurry is determined by theprocess conditions, namely temperatures, pressure and hydrated limeslurry concentration. Process conditions are optimized according tothese considerations.

Space Utilization--This design issue assesses how much the acetyleneproduction rate can be enhanced without increasing the volume of thereactor. Differing reactor configurations and process conditions willlead to differing utilization of available reactor space. In general, ifmore than 95% of the reaction is carried out in the primary reactor,then a high space utilization will be achieved.

Summary

In summary, the apparatus of the invention allows for a high level ofcontrol over mass flow and temperature and pressure in a dual reactorsystem, reducing the reactor space required to achieve a prescribedlevel of conversion, easing the fabrication requirements, providing ahigh level of recovery of acetylene produced and producing a moreconsistent and easily managed hydrated lime slurry.

The control of the invention independently maintains constant slurryconcentrations and reactor temperature, even when acetylene demand isvaried or when ambient temperatures, feed water temperatures or othertemperature effects are varied. The control of the invention allows foroperation at preferred temperature and slurry concentrations even whenthe pressure is varied.

The process of the invention maintains high recovery of acetylene byoperating at high temperatures and high slurry concentrations even whendemand for acetylene production is varied.

While this invention has been described with reference to certainspecific embodiments and examples, it will be recognized by thoseskilled in the art that many variations are possible without departingfrom the scope and spirit of this invention, and that the invention, asdescribed by the claims, is intended to cover all changes andmodifications of the invention which do not depart from the spirit ofthe invention. It will be recognized that the process conditionsdisclosed herein may not comply with local regulatory standards, andthat in practice of the invention compliance with all regulatorystandards in effect must be made.

                  TABLE I    ______________________________________    FOR A PRESSURE OF 0.05 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 20° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    350     18.00  6300    86.5   383.00                                        26.50  0.079    400     11.25  4400    84.4   437.50                                        23.19  0.106    450     7.55   3398    82.0   491.50                                        20.02  0.136    514     4.97   2555    78.7   562.78                                        18.00  0.183    540     4.41   2381    77.2   587.59                                        17.24  0.201    550     4.19   2305    76.7   598.11                                        16.94  0.209    560     3.98   2229    76.1   608.77                                        16.64  0.217    570     3.80   2166    75.5   619.20                                        16.36  0.225    600     3.30   1980    73.9   650.68                                        15.57  0.250    650     2.69   1749    71.1   702.80                                        14.41  0.292    700     2.25   1576    68.5   754.54                                        13.43  0.336    800     1.67   1334    63.7   857.26                                        11.82  0.425    900     1.31   1177    59.6   959.31                                        10.56  0.517    1000    1.07   1070    56.1   1060.96                                         9.55  0.609    1100    0.9     990    53.1   1162.37                                         8.71  0.701    ______________________________________

                  TABLE II    ______________________________________    FOR A PRESSURE OF 0.05 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 40° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    526     13.49  7098    86.7   562.85                                        18.00  0.122    610     6.84   4174    83.8   651.07                                        15.56  0.176    650     5.50   3577    82.5   692.92                                        14.62  0.202    670     5.00   3353    81.8   713.81                                        14.19  0.215    690     4.58   3162    81.2   734.68                                        13.79  0.228    730     3.90   2847    79.9   776.29                                        13.05  0.256    750     3.65   2738    79.3   797.12                                        12.71  0.269    800     3.10   2480    77.9   849.03                                        11.93  0.303    850     2.68   2278    76.5   900.73                                        11.25  0.338    900     2.35   2115    75.1   952.24                                        10.64  0.374    950     2.08   1976    73.8   1003.60                                        10.09  0.410    1000    1.87   1870    72.6   1054.83                                        9.60   0.448    1050    1.69   1772    71.4   1105.95                                        9.16   0.485    1100    1.54   1692    70.3   1156.98                                        8.76   0.523    ______________________________________

                  TABLE III    ______________________________________    FOR A PRESSURE OF 0.3 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION ON AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 20° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    450     13.50  6075    85.3   494.20                                        20.51  0.18    510     7.36   3754    81.7   553.55                                        18.3   0.23    550     5.40   2970    79.2   595.53                                        17.01  0.27    570     4.74   2702    77.9   616.18                                        16.44  0.29    600     3.97   2382    76.0   647.28                                        15.65  0.33    650     3.09   2009    72.9   698.62                                        14.50  0.38    700     2.50   1750    70.0   749.81                                        13.51  0.43    800     1.78   1424    64.8   851.26                                        11.90  0.55    900     1.38   1239    60.4   952.07                                        10.64  0.66    ______________________________________

                  TABLE IV    ______________________________________    FOR A PRESSURE OF 0.3 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 40° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    600     14.50  8700    86.8   642.20                                        15.75  0.24    650     8.98   5837    85.1   695.74                                        14.56  0.27    700     6.36   4452    83.3   748.15                                        13.54  0.32    750     4.84   3630    81.5   800.16                                        12.66  0.36    800     3.88   3104    79.8   851.98                                        11.89  0.40    850     3.20   2720    78.2   902.85                                        11.22  0.45    900     2.73   2457    76.6   954.76                                        10.61  0.50    950     2.35   2233    75.1   7005.96                                        10.07  0.54    1000    2.07   2070    73.7   1056.31                                        9.59   0.59    1050    1.84   1932    72.4   1108.32                                        9.14   0.64    1100    1.66   1826    71.2   1158.77                                        8.74   0.69    ______________________________________

                  TABLE V    ______________________________________    FOR A PRESSURE OF 1.3 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 20° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    540     11.10  5994    84.3   593.79                                        17.04  0.55    550     10.20  5610    83.8   599.76                                        16.89  0.56    560     8.80   4928    82.9   610.24                                        16.60  0.58    570     7.70   4389    82.1   620.33                                        16.33  0.60    585     6.56   3838    80.8   635.91                                        15.93  0.63    600     5.65   3390    79.6   651.03                                        15.56  0.66    650     3.88   2522    75.7   703.47                                        14.40  0.75    700     2.94   2058    72.3   752.60                                        13.46  0.85    800     1.96   1568    66.3   853.41                                        11.87  1.04    900     1.47   1320    61.5   955.66                                        10.60  1.24    ______________________________________

                  TABLE VI    ______________________________________    FOR A PRESSURE OF 1.3 BAR GAUGE    25 m.sup.3 /hr. ACETYLENE PRODUCTION AND    AT REACTOR TEMPERATURE OF 90° C.                                  Carbide                                        Ca(OH).sub.2    Fresh Water            Re-    Recycle Temper-                                  Lime  In     C.sub.2 H.sub.2  In    (t = 40° C.            cycle  Rate    ature  Out   Outlet Outlet    in Kg/hr)            Ratio  (Kg/hr) Out (° C.)                                  (Kg/Hr)                                        (%)    (Kg/hr)    ______________________________________    700     32.00  22400   88.5   748.15                                        13.54  0.67    750     11.50   8625   86.0   798.90                                        12.68  0.76    800      6.90   5520   83.7   849.83                                        11.92  0.84    850      4.90   4165   81.6   900.44                                        11.25  0.93    900      3.78   3402   79.6   951.17                                        10.65  1.01    950      3.08   2926   77.7   1001.98                                        10.11  1.10    1000     2.58   2580   76.1   1051.92                                        9.63   1.19    1050     2.23   2342   74.5   1102.29                                        9.19   1.27    1100     1.96   2156   73.1   1152.45                                        8.79   1.36    ______________________________________

                  TABLE VII    ______________________________________    ADIABATIC OPERATION FOR A PRESSURE OF 0.3 BAR GAUGE    PRIMARY REACTOR TEMPERATURE 90° C.    SECONDARY REACTOR TEMPERATURE 79.2 ° C.    INPUT WATER 20° C.    TOTAL REACTOR VOLUME 1.3 m.sup.3    EXIT LIME STREAM CONCENTRATION 17 WT. %    Acetylene            Calcium   Water          Lime   Lost    Demand  Carbide In                      In      Recycle                                     Out    Acetylene    (m.sup.3 /hr)            (Kg/hr)   (Kg/hr) (Kg/hr)                                     (Kg/hr)                                            (Kg/hr)    ______________________________________    50      178.4     1100    5940   1192   0.54    45      160.56    990     5346   1072.8 0.486    40      142.72    880     4752   953.6  0.432    35      124.88    770     4158   834.4  0.39    30      107.04    660     3564   715.2  0.32    25      89.2      550     2970   596    0.27    20      71.36     440     2376   476.8  0.22    15      53.52     330     1782   357.6  0.16    10      35.68     220     1188   238.4  0.11    ______________________________________

                  TABLE VIII    ______________________________________    ADIABATIC OPERATION EXCEPT HEATING OF INPUT    STREAMS BY EXIT LIME STREAM OR COOLING OF    RECYCLE STREAM BY COOLING WATER    RECYCLE STREAM 4000 Kg/hr    TOTAL REACTOR VOLUME 1.3 m.sup.3    PRIMARY REACTOR TEMPERATURE 90° C.    PRESSURE 0.3 BAR GAUGE    EXIT LIME STREAM CONCENTION 17 WT. %          Cal-          cium                 Re-          Car-                 cycle             Temp-    Acety-          bide                 Heat        Lost  erature    lene  in     Water   Water Loss  Lime  Acety-                                                 Sec.    Demand          (Kg/   In      In    (Kcal/                                     Out   lene  Reactor    (m.sup.3 /hr)          hr)    (Kg/hr) (° C.)                               hr)   (Kg/hr)                                           (Kg/hr)                                                 (° C.)    ______________________________________    50    178.4  1100    20.00 20960 1192  0.54  79.20    45    160.5  990     20.00 14520 1072.8                                           0.486 79.20    40    142.7  880     20.00 8120  953.6 0.432 79.20    35    124.9  770     20.00 1720  834.4 0.378 79.20    30    107.0  660     23.00 0     715.2 0.316 80.62    25    89.2   550     26.40 0     596   0.249 82.48    20    71.4   440     29.86 0     476.8 0.185 84.27    15    53.5   330     33.50 0     357.6 0.125 86.04    10    35.7   220     37.31 0     238.4 0.069 87.78    ______________________________________

                  TABLE IX    ______________________________________    ADIABATIC OPERATION EXCEPT COOLING OF    RECYCLE STREAM BY COOLING WATER    RECYCLE STREAM 4000 Kg/hr    TOTAL REACTOR VOLUME 1.3 m.sup.3    PRIMARY REACTOR TEMPERATURE 90° C.    PRESSURE 0.3 BAR GAUGE    EXIT LIME STREAM CONCENTRATION 35 WT. %          Cal-          cium                 Re-          Car-                 cycle             Temp-    Acety-          bide                 Heat        Lost  erature    lene  in     Water   Water Loss  Lime  Acety-                                                 Sec.    Demand          (Kg/   In      In    (Kcal/                                     Out   lene  Reactor    (m.sup.3 /hr)          hr)    (Kg/hr) (° C.)                               hr)   (Kg/hr)                                           (Kg/hr)                                                 (° C.)    ______________________________________    50    178.4  477.0   20.00 42300 582.5 0.236 73.06    45    160.5  431.0   20.00 36700 524.3 0.204 75.05    40    142.7  384.0   20.00 31000 465.4 0.174 76.90    35    124.9  338.0   20.00 25500 407.6 0.146 78.81    30    107.0  291.0   20.00 20400 349.1 0.119 80.63    25    89.2   244.0   20.00 15700 291.0 0.094 82.40    20    71.4   197.0   20.00 11400 233.3 0.072 84.17    15    53.5   148.5   20.00 7700  174.6 0.051 85.72    10    35.7   100.0   20.00 4470  116.6 0.032 87.33    ______________________________________

What is claimed is:
 1. An apparatus for the production of acetylene andhydrated lime by the reaction of water with calcium carbide particlescomprising;a primary reactor constructed for upward flow of water andwith an upward water velocity control flow restrictor disposed to directthe upward flow of water in the primary reactor, the restrictorconstructed and sized to allow impurities denser than hydrated limeproduced by reaction of water and calcium carbide particles in theprimary reactor to settle into a holding zone, and to entrain and carryupwards calcium carbide particles of less than a predetermined size andhydrated lime up and over an overflow at a periphery of the primaryreactor, water inlet for introducing water into the primary reactor forupward flow through the velocity control restrictor, makeup water inletfor introducing makeup water into the primary reactor, feeder forintroducing calcium carbide into the top of the primary reactor,secondary reactor concentrically disposed around the primary reactorsuch that water with calcium carbide particles and hydrated lime fromprimary reactor flowing over the overflow flow directly into thesecondary reactor, the secondary reactor constructed and dimensioned toprovide essentially plug-flow reaction of water with calcium carbideparticles and to allow the hydrated lime produced in the primary andsecondary reactors to settle into a dense zone at the bottom of thesecondary reactor,acetylene output for withdrawing acetylene produced inthe primary and secondary reactors, hydrated lime output for withdrawinghydrated lime that has settled in the dense zone from the dense zone ofthe secondary reactor, secondary water output for withdrawing water fromthe secondary reactor from a region in the secondary reactor not in thedense zone, recycle conduit directing from the secondary water output tothe water inlet of primary reactor.
 2. An apparatus as in claim 1wherein the primary reactor and the secondary reactor are enclosedwithin a common vessel capable of being pressurized.
 3. An apparatus asin claim 1 wherein the makeup water inlet is constructed to introducefresh makeup water into the upward velocity control flow restrictor. 4.An apparatus as in claim 1 wherein the makeup water inlet is constructedto introduce fresh makeup water directly into the primary reactor notthrough the velocity control flow restrictor.
 5. An apparatus as inclaim 1 additionally comprising a screen disposed above the velocitycontrol flow restrictor sized to prevent large particles fromprematurely descending through the velocity restrictor.
 6. An apparatusas in claim 1 wherein the overflow comprises a weir over which waterwith entrained hydrated lime and calcium carbide particles flow from theprimary reactor to the secondary reactor.
 7. The apparatus of claim 1wherein the secondary water output comprises a weir over which waterleaving the secondary reactor flows and which maintains a predeterminedliquid level in the secondary reactor.
 8. The apparatus of claim 7wherein the primary and the secondary reactor are enclosed within acommon pressure vessel capable of being pressurized.
 9. The apparatus ofclaim 8 wherein an annulus is formed between the secondary reactor andthe pressure vessel and water flowing over the weir flows into theannulus.
 10. The apparatus of claim 9 additionally comprising acontroller for maintaining the water level in the annulus at or above apredetermined level.
 11. An apparatus for the production of acetyleneand hydrated lime by the reaction of calcium carbide particles with anexcess of water comprising,a primary reactor for initial reaction of thecalcium carbide particles with water, a secondary reactor sized andconfigured for reaction of essentially all of the calcium carbideparticles that is not reacted in the primary reactor and such that thehydrated lime produced in the primary and secondary reactors settlesinto a dense zone in bottom of the secondary reactor, the primaryreactor concentrically disposed within the secondary reactor such thatwater with entrained calcium carbide particles and hydrated lime fromthe primary reactor flows directly into the secondary reactor via anoverflow at a periphery of the primary reactor, a hydrated lime outputfor withdrawing hydrated lime from the dense zone.
 12. An apparatus asin claim 11 wherein the primary reactor is constructed and configured toprovide a reaction environment where unreactive particles are allowed tosettle, and unreacted calcium carbide particles and hydrated limeproduced by the reaction of water and calcium carbide particles areentrained and carried to the secondary reactor.
 13. An apparatus as inclaim 11 wherein the primary and the secondary reactor are enclosedwithin a common pressure vessel capable of being pressurized.
 14. Anapparatus as in claim 11 additionally comprising a water recycle conduitfor directing water form the secondary reactor to the primary reactor.15. An apparatus as in claim 12 wherein the overflow of the primaryreactor comprises a weir over which the water with entrained calciumcarbide particles and hydrated lime from the primary reactor flowsdirectly in the secondary reactor.
 16. An apparatus as in claim 12wherein the reaction environment is provided by an entrained flow watersystem with a water flow restrictor constructed to direct a flow ofwater up through the primary reactor to create the entrained flow.